Do you know how hard it will be to build a railroad all they way across the continent? It'll be dangerous. People will die. Boilers are dangerous, the passangers will be taking their lives in there hands. No one will ever ride the thing. Trains will never be safe, it's pure folly.
Do you know how hard it will be to make a heavier than air vehicle fly? And it will be small and cramped. And it will smell bad and break down and fall out of the sky. People will die. Airplanes will never be safe, it's pure folly.
Do you know how hard it will be to transport equipment to Mars? To make a reliable life support system with sufficient backups to last long enough for the 2 year round trip? To make sustainable habitats? To develop in-situ resource utilization systems? People will die. Etc...
Why yes, we do know how hard it will be. But it's not impossible. And that's what makes it interesting. Luddites need not apply.